Field of the Invention
Embodiments of the invention generally relate to methods and apparatus for increasing blood flow and/or adjusting and maintaining the core temperature of a human.
Description of the Related Art
Homoiothermic animals, such as humans, strive to maintain relatively constant internal temperatures despite temperature variations in ambient environments and fluctuations in internal heat released as cellular metabolism byproducts. In humans, the thermal core generally includes the vital organs of the body, such as the brain and the several organs maintained within the abdomen and chest. Peripheral tissues, such as the skin, fat, and muscles, act as a buffer between the thermal core and the external environment of the animal by maintaining a temperature gradient that ranges from near-core temperature within internal organs to near-ambient temperature at the surface of the animal.
Mammalian temperature regulation requires adaptations mechanisms, such as insulation, respiratory heat conservation, and passive heat dissipation, etc., to enable mammalian survival without excessive resource expenditure to generate a stable internal thermal environment. Insulation, internal or external, impedes heat transfer from ambient condition to the body core and also protects animals from the cold. Subcutaneous insulation, similarly, retards the transfer of heat from the skin surface into the body core. The insulative properties of peripheral tissues are determined by blood flow through the tissues and in the absence of blood flow, heat transfer through the tissues is negligible. For example, lack of blood flow and poor blood perfusion makes adipose tissues good insulators. Any tissues that are poorly perfused may become insulators. Tissue blood perfusion determines local heat transfer and enables delivery of heat to (or removal from) a body region.
Respiratory heat conservation is an adaptive mechanism to prevent heat loss, heat exchange between the circulating blood and the air at the gas exchange surface of the lung alveoli in mammals. All of the circulating blood passes through the gas exchange surfaces of the lungs.
Heat is dissipated to the environment from the thermal core to the body surface by delivering through blood flow within the confines of the circulatory system. The distribution of the systemic blood is in accordance with local tissue metabolic demand. All blood passes through the chambers of the heart and the lungs. Cardiac output in a resting human is about 5 L/min so that the total blood volume circulates at a turnover rate of one cycle per minute. Blood volume and cardiac output in mammals are insufficient to uniformly perfuse all tissues in the body. Specialized vascular structures promote heat exchange in the blood flow.
Two types of vascular structures are found in mammals: nutrient vascular units and heat exchange vascular units. Their functions are mutually exclusive: The nutrient vascular units contain thin-walled, small diameter blood vessels uniformly distributed throughout the skin, such as arterioles, capillaries, and venules, and require slow blood flow through to provide nutrients to local tissues. The heat exchange vascular units contain thick-walled, large diameter venules, such as venous plexuses and Arteriovenous Anastomoses (AVAs; vascular communications between small arteries and the venous plexuses), and require flowing of large blood volumes to promote heat dissipation. In humans, the venous plexuses and AVAs of the heat exchange vascular units in humans are found mainly in the non-insulated palms of the hands, soles of the feet, ears, and non hairy regions of the face.
The thermoregulatory system in homoiothermic animals can be compromised (e.g., by anesthesia, trauma, or other factors) and may lead to the various thermal maladies and diseases. Under general anesthesia, a patient may be induced to loss the ability to conserve bodily heat. Thermal maladies, such as hypothermia and hyperthermia, can occur when the thermoregulatory system is overwhelmed by severe environmental conditions. Constriction of the AVAs thermally isolates the body core from the environment, while, dilation of the AVAs promotes a free exchange of heat between the body core and the environment.
Blood flow through the heat exchange vascular structures can be extremely variable, for example, high volume of blood flow during heat stress or hyperthermia can be increase to as high as 60% of the total cardiac output. Hypothermia, on the other hand, is the result of prolonged exposure to a cold challenge where blood flow through the venous plexuses and AVAs can be near zero of the total cardiac output. Vasoconstriction of the peripheral blood vessels may arise under hypothermia in order to prevent further heat loss by limiting blood flow to the extremities and reducing heat transfer away from the thermal core of the body. However, vasoconstriction makes it much more difficult to reverse a hypothermic state since vasoconstriction impedes the transfer of heat from the body surface to the thermal core and makes it difficult to simply apply heat to the surface of the body. This physiological impediment to heat transfer is referred to as a vasoconstrictive blockade to heat exchange. There is a need to regulate blood flow to the venous plexuses and AVAs of the heat exchange vascular units and intervene thermal maladies.
Other thermal malady related diseases, such as venous thromboembolic disease, continues to cause significant morbidity and mortality. Hospitalization due to venous thrombosis and pulmonary embolism (PE) ranges from 300,000 to 600,000 persons a year. Following various types of surgical procedures, as well as trauma and neurological disorders, patients are prone to developing deep vein thrombosis (DVT) and PE, which usually originate from blood clots in the veins and some clots traveling to the lung. Regardless of the original reasons for hospitalization, one in a hundred patients upon admission to hospitals nationwide dies of PE. Patients suffering from hip, tibia and knee fractures undergoing orthopedic surgery, spinal cord injury, or stroke are especially at high risk. Thus, prevention of DVT and PE is clinically important.
It is believed that slowing of the blood flow or blood return system from the legs may be a primary factor related to DVT with greatest effect during the intraoperative phase. Also of concern is the postoperative period. Even individuals immobilized during prolong travel on an airplane or automobile may be at risk. Generally, without mobility, return of the blood back to heart is slowed and the veins of an individual rely only on vasomotor tone and/or limited contraction of soft muscles to pump blood back to the heart. One study shows that travel trips as short as three to four hours can induce DVT and PE.
Current approaches to prophylaxis include anticoagulation therapy and mechanical compression to apply pressure on the muscles through pneumatic compression devices. Anticoagulation therapy requires blood thinning drugs to clear clots in the veins which must be taken several days in advance to be effective. In addition, these drugs carry the risk of bleeding complications. Pneumatic compression devices, which mechanically compress and directly apply positive message-type pressures to muscles in the calf and foot sequentially, are not comfortable, are difficult to use even in a hospital, and are too cumbersome for mobile patients or for use during prolonged travel. In addition, most of them are heavy weighted and there are no portable or user friendly devices.
U.S. Pat. No. 5,683,438, issued to Grahn and assigned to Stanford University, discloses an apparatus and method for overcoming the vasoconstrictive blockade to heat exchange by mechanically distending blood vessels in a body portion and providing heat transfer to the body core of a hypothermic mammal. The disclosed device comprises a fluid-filled heating blanket that is lodged within a tubular, elongated hard shelled sleeve placed over the body portion. Sub-atmospheric pressure is applied and maintained within the sleeve. However, most devices for regulating body temperature may not provide sufficient heat or adequate surface area for heat transfer being optimized and evenly distributed between the heating element and the body of the patient. In addition, the devices may not be able to adapt to the variability in patient sizes or provide mobility of the body portion during prolong treatment.
Therefore, there remains a need for an apparatus and method to increase blood flow to the venous plexuses and AVAs of the heat exchange vascular units, thereby reducing the vasoconstrictive blockade and promoting heat exchange for body temperature regulation and disease intervention.